The present invention relates to geophysical surveying and more particularly to improved methods and apparatus for radio geophysical surveying.
For many years geologists involved in geophysical surveying have relied exclusively upon data obtained by direct observation. Most of such information came from natural exposures, although some facts were revealed by artificial openings such as mines, tunnels, and railroad and highway cuts. In the last few decades, however, geophysical methods have become highly developed, chiefly under the impetus of the petroleum and mining industries. By the use of geophysical methods, especially when combined with existing geological information, it is possible to determine many of the physical properties of subsurface formations, even to depths as great as 10,000 feet and deeper.
Among the many geophysical methods are radio geophysical methods. Methods and apparatus for sending electromagnetic energy at radio frequencies into the earth and receiving reflections thereof with changes in intensity and/or phase noted are common in the prior art. The changes in the returned energy are indicative of the types of subterranean media through which the energy has travelled. Variations in the changes are indicative of different types of formations, faults, or mineral deposits.
It is generally recognized in radio geophysics that the incident radio waves are changed due to the electrical properties of intervening media. These properties include principally the conductivities, the magnetic permeabilities, and the dielectric constants of the various strata and deposits therein. Further, the radio waves are affected by the size of a deposit, its depth below the surface, and its orientation.
In order to practice a radio geophysical method, it is necessary to generate radio waves, propagate them into the earth, receive waves reflected from the formations within the earth, and measure certain parameters of the reflected waves. The most common parameters of interest are the magnitude and the phase of the reflected waves. A common manner of measuring these parameters is to transmit a reference wave or signal that is surface propagated and, in the receiver, to separate the effects of the reflected wave from the effects of the surface wave. A widely used technique for separating the surface and earth propagated waves and the effect thereof is known as polarization diversity, wherein the surface waves and the earth waves are given different polarizations. The waves may be received on antennas connected to separate receivers, each antenna having a different response polarization, and the parameters of the reflected waves may be measured with respect to the surface waves.
The polarization direction of an electromagnetic wave is the direction of the electric field vector thereof. It has been found that a wave must be horizontally polarized, that is the electric field vector must be horizontal for the wave to be propagated into the ground. One reason for this is that a horizontally polarized wave may have magnetic field intensity vector components in all directions perpendicular to the electric field vector. The propagation direction of an electromagnetic wave is in a direction perpendicular to the mutually perpendicularly electric and magnetic field vectors. For the horizontal electric field vector with a horizontal magnetic field vector, the direction of propagation is vertical. In contrast, a vertically polarized wave with a vertical electric field vector could only have a horizontal magnetic field vector. The direction of propagation would, consequently, be horizontal; therefore, the vertically polarized wave would not be propagated into the earth.
Since a vertically polarized signal is propagated principally in a horizontal direction and not into the ground where changes in its magnitude and phase might occur, a vertically polarized wave is generally employed as a reference signal. Since a horizontally polarized signal can be propagated into the ground, such a polarized signal is employed for determining characteristics of subterranean formations.
In most radio geophysical methods, relative magnitude and phase of reflected waves are measured at each of a plurality of locations along a survey path. Data points are then plotted as a function of position and connected by a continuous curve. The variations of the curves in relation to position are then indicative of the electrical nature of the earth at each position.
One radio geophysical apparatus, as disclosed in U.S. Pat. No. 2,994,031, issued to Slattery, comprises a transmitter connectible to either a vertical antenna or a loop antenna and a pair of receivers, one connected to a vertical antenna and the other connected to a loop antenna, a phase shifter, and phase and volt meters. During transmission on the vertical antenna, the receivers each receive a signal. The signal received by the vertical receiver is adjusted in magnitude and phase and added to the signal received by the loop receiver whereby cancellation or nulling occurs. Then the loop is connected to the transmitter, and a signal is transmitted. The part of the transmitted signal originating from the horizontal arms of the loop antenna is horizontally polarized and is propagated into the ground, reflected back, and received by the loop receiver. The vertically polarized component, from the vertical arms of the loop antenna, is surfaced propagated and is received by the vertical receiver and by the loop receiver. The receivers are adjusted in an attempt to null the vertically polarized signal components, such that the remaining signal in the receivers is, for the most part, due to the reflected signal components.
A very important requirement in a radio geophysical surveying method is consistency throughout the survey, and from survey to survey. Otherwise, useful comparison of plots derived from various surveys is impossible. Not only should the transmission power level and frequency be consistent, but also the receiver stage gains, spacing between the transmitting and receiving antennas, and the orientation of the antennas. A further aid to consistency is automation of the survey routine to the extent possible in order to diminish human error.
Some of the prior art radio geophysical survey methods and devices apparatus, including that of U.S. Pat. No. 2,994,031 mentioned above, require the operator to make manual adjustments for nulling the effect of the reference signal in the receivers. Others require the visual reading of meters for data taking. Such actions inherently involved inaccuracies, and during a survey wherein a great many adjustments and readings must be made, operator fatigue seriously affects the accuracy of the survey.
The methods and apparatus of the present invention have application in surveying for geologic faults, hydrocarbon deposits, and other mineral deposits. Petroleum deposits are generally accompanied by a gas cap or "halo" comprising mixtures of methane, ethane, and propane in porous strata surrounding the strata containing petroleum. These gases are known to be paramagnetic; that is, they have a relative permeability greater than that of a vacuum, which is unity. Petroleum itself is dielectric. Therefore, an electromagnetic wave incident upon a petroleum deposit is changed because of the difference in electrical properties of the petroleum and gas from the surrounding media. One of the changes which occurs in an apparent amplification of the transmitted waves which occurs with the transmitting and receiving antennas in certain mutual orientations. One theory is that the amplification is caused by a combination of effects of the magnetic field components of the transmitted waves and the earth's magnetic field on the paramagnetic gases.
By way of explanation, in a paramagnetic substance the atoms form small dipole type molecules. The molecules are normally randomly oriented with respect to each other. If a finite magnetic field is applied, the dipoles become aligned in the direction of the magnetic field. Thus, energy from the earth's magnetic field is stored in the dipoles. When these paramagnetic materials are subjected to an RF wave of proper frequency, in the presence of a finite magnetic field, the dipoles tend to self-oscillate in a resonance mode, thus imparting energy into the reflected wave and acting like an amplifier. The energy is imparted to the wave by way of the magnetic field vector thereof. It is known that current may be induced in a conductor, such as en element of an antenna, by either the electric field vector of a wave or by the magnetic field vector or by a combination of the two according to the orientation of the conductor. Therefore, by placing a transmitting antenna and a receiving antenna in an orientation such that the current in the receiving antenna is derived from the magnetic field vector, changes in a wave caused by paramagnetic substances can be detected.
In a similar manner, the electric field vector of a wave is affected by the dielectric nature of a substance such as petroleum. Therefore, a mutual transmitting and receiving antenna orientation, wherein the current in the receiving antenna is derived from the electric field vector, may be employed for detecting changes in a wave caused by dielectric substances.
Heretofore, the detection of effects on the electric and magnetic field vectors was generally accomplished by resolving a received signal into resistive, inductive, and capacitive components or, in other terms, in-phase and quadrature leading or lagging components. In most such systems, only the resistive or in-phase components are of interest because the systems are directed to prospecting for conductive ores, and the non-resistive information is discarded as being of no use. The method of the present invention includes the taking of data with the transmitting and receiving antennas in both an "electric" orientation and a "magnetic" orientation, whereby no resolution of resistive, inductive, and capacitive components is required. The method of the present invention, therefore, has application, without modification, in surveying for many types of subterranean formations.